use headline_prefix method generally. (move from html/latex builder to base builder.)
* lib/review/idgxmlbuilder.rb, test/test_idgxmlbuilder.rb:
  stop providing a index from @<ttb>.
  That behavior just makes a confuse.
* test/sample-book/src/config.yml

improve config.yml to reference 'doc/libepubmaker/config.yml'
Merge pull request #346 from akinomurasame/improve-config-yml

Improve test/sample-book/src/config.yml
Gemfile.lock makes a dirty status
Raise errors if no latex tools are installed
implement list_header() and list_body() in markdownbuilder.rb
Merge pull request #358 from gfx/markdown/list

implement list_header() and list_body() in markdownbuilder.rb
Merge pull request #356 from gfx/no_gemfile_lock

Gemfile.lock makes a dirty status
Merge branch 'raise_if_no_latex_installed' of https://github.com/gfx/review into gfx-raise_if_no_latex_installed
fix #357; allow to use original system() and customized system()
Merge branch 'master' of github.com:kmuto/review